1. Field of the Invention
The present invention relates to isolation mounts for use in vacuum chambers. More particularly, the invention relates to the application of such devices in lithographic projection apparatuses.
2. Discussion of Related Art
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The radiation system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, and such elements may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively. Further, the lithographic apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatuses are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
Lithographic projection apparatuses can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed v at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In a lithographic apparatus, the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices and hence higher operating speeds, it is desirable to be able to image smaller features. Whilst most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include laser plasma sources or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in xe2x80x9cSynchrotron radiation sources and condensers for projection x-ray lithographyxe2x80x9d, J B Murphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993).
Other proposed radiation types include electron beams and ion beams. These types of beam share with EUV the requirement that the beam path, including the mask, substrate and optical components, be kept in a high vacuum. This is to prevent absorption and/or scattering of the beam, whereby a total pressure of less than about 10xe2x88x926 millibar is typically necessary for such charged particle beams. Wafers can be contaminated, and optical elements for EUV radiation can be spoiled, by the deposition of carbon layers on their surface, which imposes the additional requirement that hydrocarbon partial pressures should generally be kept below 10xe2x88x928 or 10xe2x88x929 millibar. Otherwise, for apparatuses using EUV radiation, the total vacuum need pressure only be 10xe2x88x923 or 10xe2x88x924 mbar, which would typically be considered a rough vacuum.
Further information with regard to the use of electron beams in lithography can be gleaned, for example, from U.S. Pat. No. 5,079,122 and U.S. Pat. No. 5,260,151, as well as from EP-A-0 965 888.
Working in such a high vacuum imposes quite onerous conditions on the components that must be put into the vacuum and on the vacuum chamber seals, especially those around any part of the apparatus where a motion must be fed-through to components inside the chamber from the exterior. For components inside the chamber, materials that minimize or eliminate contaminant and total outgassing, i.e. both outgassing from the materials themselves and from gases adsorbed on their surfaces, should be used. It would be very desirable to be able to reduce or circumvent such restrictions.
It is an object of the present invention to provide an improved isolation mount that can be used in a vacuum chamber of a lithographic projection apparatus, for example to support a metrology frame.
According to the present invention, this and other objects are achieved in a lithographic projection apparatus that has a radiation system for supplying a projection beam of radiation; a first object table provided with a mask holder for holding a mask; a second object table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate. The lithographic projection apparatus also has a vacuum chamber having a wall enclosing at least one of the first and second object tables, the vacuum chamber wall having at least one aperture therein; a reference frame provided in the vacuum chamber; a position sensing device for sensing the position of an object in the vacuum chamber relative to the reference frame; and a support member passing through the aperture to support the reference frame from outside the vacuum chamber and to isolate the reference frame from the vacuum chamber wall.
Current lithography apparatuses are designed for use in clean room environments and therefore some steps have conventionally been taken to reduce possible sources of contamination of wafers that are processed by the apparatus. However, conventional designs of wafer, mask and transfer stages are very complicated and employ large numbers of components for sensor and drive arrangements. Such stages also need to be provided with large numbers of signal and control cables and other utilities. The present invention avoids the difficult and detailed task of making such large numbers of components vacuum-compatible, or replacing them with vacuum-compatible equivalents, by adopting the principle of locating as many components and functions as possible outside the vacuum chamber. The present invention thus avoids the need to vacuum-proof many or most of the numerous components, by providing appropriate mechanical feed-throughs with innovative sealing arrangements. Likewise, the present invention avoids difficulties in reducing vibrations inevitable in vacuum apparatuses particularly where powerful pumps are provided, by isolating as far as possible vibration sensitive components from the vacuum chamber wall.
By supporting the reference frame from outside the vacuum chamber and isolating it from vibrations in the vacuum chamber wall, such vibrations are prevented from degrading the accuracy with which the object tables are positioned and moved, and the reliability of position measurements made relative to the reference frame.
According to another aspect of the invention, a lithographic projection apparatus has a radiation system for supplying a projection beam of radiation; a first object table provided with a mask holder for holding a mask; a second object table provided with a substrate holder for holding a substrate; and a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate. The lithographic projection apparatus of this preferred embodiment has a vacuum chamber having a wall enclosing at least one of the first and second object tables, the vacuum chamber wall having at least one aperture therein; a reference frame extending through the aperture so as to have a first part located in the vacuum chamber and a second part located outside the vacuum chamber; a sensing device having a first component mounted to the first part of the reference frame and a second component mounted on the second part of the reference frame; and a low-stiffness seal for sealing between the vacuum chamber wall and the reference frame.
This arrangement allows the relative positions of two components mounted on the reference frame, one inside and one outside the vacuum chamber, to be maintained constant irrespective of any deformation of the vacuum chamber walls with changes in the pressure differential across them.
A further aspect of the invention is directed to a low-stiffness seal for sealing a space between a vacuum chamber wall and an elongate rod having a longitudinal axis and passing through an aperture in the vacuum chamber wall. The seal includes an annular collar rotatably mountable around the elongate rod and having a first sealing surface concentric with the longitudinal axis;
an intermediate member having second and third sealing surfaces, the second sealing surface being complementary to and opposing the first sealing surface; and a plate member defining an aperture through which the elongated rod passes and a fourth sealing surface complementary to and opposing the third sealing surface, the plate member being mountable to the pressure chamber wall.
The low-stiffness seal of the invention can provide a seal about a rod passing into a vacuum chamber whilst allowing movement of the rod relative to the vacuum chamber wall in six-degrees of freedom.
In a preferred embodiment of the invention, the low-stiffness seal is used to seal around a support pillar of a reference frame provided in a vacuum chamber of a lithographic apparatus.
Another aspect of the invention is directed to a device manufacturing method that includes providing a substrate which is at least partially covered by a layer of energy-sensitive material; providing a mask containing a pattern; projecting an image of at least part of the mask pattern onto a target area of the layer of energy-sensitive material using a projection beam of radiation.
At least one of the substrate and the mask is provided in a vacuum chamber; and the one of the mask or the substrate is positioned relative to a reference frame provided in the vacuum chamber, the reference frame being supported from outside the vacuum chamber and isolated from the walls of the vacuum chamber.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping) metallisation, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.